This invention relates to fuel cells, and more particularly to liquid-cooled fuel cells having electrically insulated coolant manifold(s) to reduce shunt currents in the coolant.
Fuel cells have been proposed as a power source for a variety of applications. Some fuel cells (e.g. PEM-type or phosphoric-acid-type) use hydrogen supplied to the anode as fuel, and oxygen (as air) supplied to the cathode as oxidant. PEM (i.e. Proton Exchange Membrane) fuel cells are preferred for vehicular applications owing to their compactness, moderate temperature operation, and high power density. PEM fuel cell stacks comprise a plurality of individual cells each of which has a so-called xe2x80x9cmembrane-electrode-assemblyxe2x80x9d comprising a thin, proton-transmissive, solid polymer, membrane-electrolyte (e.g. perfluronated sulfonic acid) having an anode on one face of the membrane-electrolyte and a cathode on the opposite face of the membrane-electrolyte. The number of cells in any given stack is determined by the desired output voltage of the stack. The anode and cathode typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles. The membrane-electrode-assembly is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, and contain flow channels on their faces for distributing the fuel cell""s gaseous reactants (i.e., H2 and O2/air) over the surfaces of the respective anode and cathode. One such membrane-electrode-assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 to Swathirajan et al. and assigned to the assignee of the present invention.
Bipolar PEM fuel cell stacks comprise a plurality of the membrane-electrode-assemblies stacked together (typically in electrical series), and separated one from the next by a gas-impermeable, electrically-conductive bipolar plate. Each bipolar plate has an active region having a first face confronting the anode of one cell, a second face confronting the cathode of the next adjacent cell in the stack, and an internal cooling passage for circulating a coolant (e.g. ethylene glycol and water) through the plate behind the faces. Monopolar end plates are provided at the ends of the stack. An inactive region borders the active region of the bipolar plate, and has one or more openings therein. When the plates are stacked together, like openings in adjacent plates are aligned, and together with other stack components (e.g. gaskets), they form inlet and outlet manifolds that respectively supply and remove the gaseous reactants and coolant to/from the several bipolar plates. When so aligned a surface of each of the bipolar plates that defines the coolant opening forms part of the wall that defines the coolant manifold. This surface is electrically conductive, and during operation of the fuel cell contacts the coolant. This contact causes unwanted, parasitic shunt currents to flow through the coolant, and consequent reduced stack efficiency and possible electrolytic degradation of the coolant.
The present invention reduces the shunt currents that flow through the coolant in a fuel cell stack by electrically insulating the walls of the manifolds that supply and remove coolant from the stack. A fuel cell stack is provided that comprises a plurality of cells each of which has an anode exposed to a first reactant, a cathode exposed to a second reactant and an electrolyte interjacent the anode and cathode. The stack includes at least one (typically many) electrically conductive, bipolar plate that separates adjacent cells from each other. The bipolar plate comprises an electrically-conductive active region that has a first face confronting the anode of one cell, a second face confronting the cathode of the next adjacent cell in said stack, and an internal cooling passage for circulating a coolant through the plate behind the faces. The active region is bordered by at least one inactive region that houses manifolds for conducting the coolant and the reactants to/from the stack. In this regard, the inactive region of each bipolar plate has a surface that defines an opening through the inactive region, and that, in part, defines a manifold that supplies or removes coolant respectively to or from the coolant passage within the bipolar plate. In accordance with the present invention, at least the opening-defining surface of the inactive region is coated with an adherent, non-conductive coating that reduces the flow of shunt currents through the coolant in the manifold. Preferably, the entire inactive area of the bipolar plate is coated with the non-conductive coating for additional protection and ease of coating. The non-conductive coating preferably comprises a polymer, or, most preferably, an oxide of the metal used to make the bipolar plate, and may be applied by spraying, brushing, dipping, electrolytically, CVDing or PVDing. When the bipolar electrode is made from a metal such as titanium, the non-conductive coating will preferably comprise an oxide (i.e. titanium oxide) of that metal formed in situ (e.g, by anodization). The invention is particularly advantageous to PEM fuel cells that have proton-transmissive membrane electrolytes, rather than a flowing liquid electrolyte through which shunt currents can flow.